Devices of this type for recovery of potential energy in working machines are prior art; see, for example, WO 93/11363 or EP 0 789 816 B1. As energy storage systems, such devices have pressure accumulators storing the released potential energy as pressure energy of a working gas. It is crucial for the efficiency of these devices that the lowest possible energy losses occur in operation. The losses involve primarily losses of thermal energy of the accumulator gas. Generally, a large part of the thermal energy forming when the working gas is compressed is released via the outer walls of the hydraulic accumulator used in the prior art as an energy storage system. The large-area contact region between the working gas and the exterior can lead to considerable heat losses for the relatively large surface of the accumulator housing (preferably of steel) under consideration.